Georgia's Supreme Court halted Monday evening's scheduled execution of a convicted murderer on procedural grounds, but rejected a plea to spare him due to mental retardation.

The unanimous ruling from the seven-member court gave a reprieve to 52-year-old Warren Lee Hill, who was condemned for bludgeoning a fellow inmate to death in 1990. He had been scheduled to die at 7 p.m. Monday.

Justices said they would review whether prison officials needed public hearings before changing the drugs used in lethal injections from a mix of three to a single drug.

Hill's lawyers also argue that the condemned convict should be spared because he has an IQ of 70. The U.S. Supreme Court in 2002 struck down the death penalty for convicts with mental retardation, but Georgia law says an inmate has to prove mental retardation beyond a reasonable doubt, and a state court ruled earlier this month that Hill's lawyers had failed to do that.

All justices except Robert Benham refused to review Hill's appeal of that case.

"I cannot join in any ruling by this court that would allow the execution of Warren Hill Jr., who has been found by a preponderance of the evidence to have a mental disability," Benham found.

Hill was already serving a life term in the 1985 shooting death of his girlfriend, Myra Sylvia Wright, when convicted of killing inmate Joseph Handspike.

The state announced last week that it was replacing its use of a three-drug cocktail to put prisoners to death with a single drug, pentobarbital. Hill's lawyers argued the decision required public notice under the state's Administrative Procedure Act, a question the court halted proceedings to consider.